1. Field of the Invention
The present invention relates to a composition of electrolytic manganese dioxide-containing cathodic active material used as a cathodic active material for dry cells, in particular, alkaline manganese dioxide cells as well as a method for the preparation thereof. The present invention also relates to a dry cell which makes use of the foregoing composition.
2. Prior Art
Manganese dioxide has long been used, as a cathodic active material for dry cells. Manganese dioxide products used for this purpose can roughly be classified into three groups, i.e., natural manganese dioxide, chemically synthesized manganese dioxide and electrolytic manganese dioxide and are properly used depending on purposes.
Among these manganese dioxide products, electrolytic manganese dioxide provides a dry cell which has an excellent discharge quality at a high load. In addition, the ratio of dry cells using electrolytic manganese dioxide has gradually been increased since small dry cells having high capacities have recently been desired along with the miniaturization of electronic machinery and tools.
There has been increased applications of a small-sized dry cell such as the LR-6 type which can be discharged at an extremely high load such as discharge at a load of 1.OMEGA..
The electromotive force of the dry cell composed of electrolytic manganese dioxide presently used reveal a substantial drop when the cell is discharged at a load of 1.OMEGA., which results in a reduction of the capacity of one third the theoretical level.
Many attempts have already been made to improve the discharge quality of dry cells at a high load. For instance, Japanese Unexamined Patent Publication (hereunder referred to as "J. P. KOKAI") No. Hei 02-213487 and J. P. KOKAI No. Sho 60-138085 proposes that electrolytic manganese dioxide showing excellent properties in high load-discharge can be produced by carrying out the electrolysis in an electrolyte in which carbon fibers or carbon particles are suspended. In addition, J. P. KOKAI No. Hei 05-9773 proposes that a high quality electrolytic manganese dioxide product can be obtained by dispersing, in a manganese electrolyte, carbon fibers, and/or graphite fibers whose surface is covered with a film of a manganese oxide and by co-precipitating the fibers and manganese dioxide through electrolysis to thus obtain precipitates having a manganese dioxide content of not less than 90% by weight. Moreover, J. P. KOKAI No. Sho 63-21224 discloses a method for preparing high quality electrolytic manganese dioxide in which the electrolysis is carried out in an electrolyte suspending fine particles of manganese oxide.
The aforementioned conventional techniques direct improvement in characteristic properties of electrolytic manganese dioxide per se through appropriate adjustment of conditions for the electrolysis. Alternatively, other attempts have also been done, in which the processes for preparing a cathodic composite material subsequent to the electrolysis are improved. For instance, J. P. KOKAI Nos. Hei 03-1444, Hei 03-11554 and Hei 03-47196 disclose that the high load-discharge quality of manganese dioxide cells whose cathodic active material comprises a mixture of chemically synthesized manganese dioxide and electrolytic manganese dioxide can be highly improved as compared with the cells whose cathodic active material comprises only electrolytic manganese dioxide. In addition, J. P. KOKAI No. Sho 62-103973 discloses that high load-discharge quality of a manganese dioxide cell can be improved through the use of a electrolytic manganese dioxide which is pulverized twice, first with the usual pulverization and then again after compression. Furthermore, J. P. KOKAI No. Sho 57-27929 discloses that characteristic properties of dry cells can be improved if precipitated manganese dioxide particles are immersed in a diluted sulfuric acid solution, then neutralized and used as the cathodic active material of the cells. J. P. KOKAI No. Sho 63-21225 sets forth that characteristic properties of dry cells can be improved through the use of manganese dioxide obtained by the next process, electrolysis, rough and fine pulverization, neutralization, and adding an oxidizing agent thereto. J. P. KOKAI No. Sho 63-40727 teaches that fine particles having a size of not more than 2 .mu.m and coarse particles having a size of not less than 92 .mu.m are inferior in quality and that accordingly, quality of dry cells can be improved through the use of electrolytic manganese dioxide from which these fine and coarse particles are removed in advance.
On the contrary, J. P. KOKAI No. Sho 51-21125 discloses that discharge quality of dry cells can be improved by controlling the average particle size of electrolytic manganese dioxide used to not more than 5 .mu.m. Moreover, J. P. KOKAI No. Sho 51-21129 discloses that discharge quality can be improved through the combination of improvement in the electrolyte used and the use of electrolytic manganese dioxide having an average particle size of not more than 5 .mu.m. Further, J. P. KOKAI No. Sho 58-14470 discloses that high load-discharge quality of dry cells can be improved through the use of electrolytic manganese dioxide having an average particle size of not more than 10 .mu.m in an electrolyte containing sodium perchlorate.
Moreover, J. P. KOKAI Nos. Hei 02-195647 and Hei 02-226656 set forth that the use of carbon black obtained by the furnace process as a conductive material permits improvement of dry cells in their high load-discharge quality. On the other hand, J. P. KOKAI No. Sho 63-121256 discloses that conductive properties of a cathodic material is improved by forming a thin film of a carbonaceous material on the surface of manganese dioxide particles and that a manganese dioxide cell having high load-discharge quality can thus be produced. J. P. KOKAI No. Sho 63-187570 teaches that the high load-discharge quality of a dry cell can be improved through the use of electrolytic manganese dioxide particles which are covered with fine particles of a carbonaceous material having an averaged particle size ratio, with respect to the manganese dioxide particles, ranging from 10.sup.-1 to 10.sup.-3 at a covering rate ranging from 0.5 to 15%. However, this invention mainly aims at the improvement of conductivity and, therefore, the particle size of the carbonaceous material should be smaller than that of manganese dioxide.
Various attempts have been proposed as has been discussed above, but all of them are still insufficient since they suffer from various problems in, for instance, that the effect attained is insufficient, that the practical production of the product proposed therein is very difficult and that mass-production of such products is impossible. Thus, there is much room for further improvement.
More specifically, electrolytic manganese dioxide should receive electrons for inducing cell discharge. This, in turn, requires migration of protons or electrons through the electrolytic manganese dioxide. However, the manganese dioxide serving as the path for electron or proton-migration is deteriorated since the rate of migrating protons or electrons per unit time is substantially increased during the high load-discharge. This leads to inhibition of proton or electron-migration and hence a reduction of electromotive force of the dry cell. Accordingly, it is believed that the particle size of electrolytic manganese dioxide particles is favorably as low as possible for the discharge at a high load.
In the conventional methods, however, if electrolytic manganese dioxide is strongly pulverized to reduce the particle size thereof, electrolytic manganese dioxide is deteriorated and accordingly, quality thereof is rather impaired, while if the particle size of the manganese dioxide particles is reduced, but aggregates thereof still remain, particles of a conductive material such as graphite are not sufficiently brought into close contact with each other and accordingly, any effect of the particle size reduction cannot be anticipated. In addition, if fine particles are used, any increase in density is not ensured when the fine particles are subjected to compression powder molding and this accordingly results in the reduction in the amount of the particles capable of being packed in a predetermined volume in a dry cell.